The present invention relates to noise attenuation systems. In particular, the present invention relates to noise attenuation systems for use with gas turbine engines such as aircraft auxiliary power units.
Large commercial aircraft typically include on-board auxiliary power units, located in the tail sections of the aircraft, to provide electrical power and compressed air for systems throughout the aircraft. When an aircraft is on the ground, the primary propulsion engines of the aircraft are shut down, and the auxiliary power unit provides the main source of power for a variety of systems, such as the environmental control systems, hydraulic pumps, electrical systems, and main engine starters. The auxiliary power unit may also provide power during in-flight operations, such as for electrical and pneumatic systems.
In many gas turbine engine applications, particularly those in which the engine is used in conjunction with a commercial passenger aircraft, there is a widespread demand by the airline industry to maintain noise levels below defined limits. This is particularly important at ground service stations for the aircraft, where ground crew load and unload luggage, fuel and provision the aircraft, and remove waste materials from the aircraft. Under these conditions, the aircraft auxiliary power unit is the turbine engine of interest.
Noise generated during the operation of the auxiliary power unit typically includes low frequency noise generated during the combustion process within the turbine engine, and high frequency noise generated by the mixing of gases at the compressor portion of the turbine engine. The low frequency noise is typically attenuated with an exhaust silencer placed downstream from the auxiliary power unit. High frequency noise is typically attenuated in other manners.
The arrangement of the exhaust silencer downstream of the auxiliary power unit allows the exhaust silencer to dissipate acoustic energy of the low-frequency noise as the noise travels downstream from the exhaust diffuser. However, positioning the exhaust silencer downstream from the exhaust diffuser exposes the exhaust silencer to the exhaust gases emitted from the auxiliary power unit. The exhaust gases can convectively heat the exhaust silencer to elevated temperatures above the auto-ignition temperature of the fuel (e.g., greater than 600° C.). Such elevated temperatures can degrade the structural integrity of the exhaust silencer, thereby requiring the use of high-temperature materials and insulation layers. High-temperature materials and insulation layers, however, undesirably increase the cost and weight of the exhaust silencer.